Have multiple phones using Zoiper with Verizon and TMobile for our business (technicians in the field, want the main phone line to ring everyone). Got a new employee who uses AT&T and cannot get Zoiper to register when he is on AT&T network. Works fine in his home when on WiFi or in our store on WiFi, but not with network. Not seeing a lot in online searches.

Checked his credentials by using them on another network phone (Verizon), and it registered just fine.

Using IPytomy PBX.

Samsung S8+

Zoiper 2.7.12

Anyone know of any settings with AT&T that may prevent Zoiper from registering?

I believe there were similar complaints previously from AT&T's network. I can suggest that you ask the user to switch the transport to TLS and SRTP for the audio(if your PBX is supporting it). This will workaround potential NAT issues.